[ARC5] Cool New DC-DC Converter

[Bob kb8tq]

Hi

The whole “less is better” approach is what we had been using across the board on parts.
50% max was our magic number. Went into the design review and started going through 
everything. Made it to parts and one guy’s eyes lit up. He reached into a folder and pulled
out a coupe of studies. (Odd that he just *happened* to have them along with him …). 
They pretty well showed that on some caps, you get a bathtub curve for reliability vs voltage.
Go to low and the reliability starts to degrade again …… surprised the heck out of all of
us (to say the least). 

Bob

> On Mar 18, 2018, at 12:31 PM, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:
> 
> The failure of an electrolytic cap vs how close to its rating it is run is a soft function.  It's not like the sudden failure of a solid dielectric cap.  It takes a long time.
> 
> To get a decent lifetime a 'lytic should always be run under its rating, just how much depends upon how much reliability you are after.  HP was conservative; their design guidelines were 60%.  Most other makers were less conservative in order to keep cost and size down.  My vintage ham gear seems to apply roughly 80% and sometimes more.  They also don't respect ripple heating enough, mounted high wattage bleeders close by and increasing moved toward ESR hostile cap input filters with solid state rectification.
> 
> The other factor is dry-out due to age and heating.  The higher the ESR and the higher the ripple, the more self heating.  The more heating, the more drying. The effect is a very slow runaway condition that eventually manifests in cap failure.
> 
> I you buy a modern 105C-rated low-ESR cap for replacement and don't try to squeeze the last volt out of it, it's going to reward you with long life.  They are built to survive in abusive PWM circuits.  Our circuits are a cakewalk by comparison.
> 
> Dennis AE6C
> 
> 
> 
> On Sun, Mar 18, 2018 at 9:12 AM, Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
> Hi
> 
> The real point is that a “250V” cap may or may not have a max rating of exactly 250.0V.
> It may indeed have an actual max voltage on that sample a bit higher than 250V. These
> days it’s very much a “who knows” sort of thing.
> 
> Oddly enough the same rule that says “don’t use the last 10     %” also says “don’t use less
> than 60%” on an electrolytic cap. Learned that one in a design review …. on product headed
> for space no less …. gulp ….
> 
> Bob
> 
> > On Mar 18, 2018, at 12:06 PM, Scott Robinson <spr at earthlink.net <mailto:spr at earthlink.net>> wrote:
> >
> > Well, a switching power supply designer who is even older than I am told me that if you avoid the using the last 10% or so of an electrolytic cap's voltage rating that it will last longer.
> >
> > FWIW,
> >
> > Scott Robinson
> >
> > On 3/18/18 7:33 AM, Bob kb8tq wrote:
> >> Hi
> >>
> >> In some cases, indeed a 250V cap is quite happy for a very long time at 300V. It’s not
> >> all “great balls of fire” sort of stuff ….
> >> Bob
> 
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